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CH4 mixture
Journal of Natural Gas Science and Engineering 26 (2015) 1246e1253
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Performance evaluation of clinoptilolite and 13X zeolites in CO2 separation from CO2/CH4 mixture A. Arefi Pour, S. Sharifnia*, R. NeishaboriSalehi, M. Ghodrati Catal. Res. Cen., Dept. Chem. Eng., Razi Univ., Kermanshah 67149-67246, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2015 Received in revised form 14 August 2015 Accepted 17 August 2015 Available online 19 August 2015
In this study, the adsorption performance of clinoptilolite and 13X zeolites as adsorbent was examined to remove CO2 from CO2/CH4 mixture. The adsorption experiments of CO2 and CH4on 13X and clinoptilolite zeolites were conducted at 277, 290 and 310 K for pressures up to 10 bar. The adsorption capacity of CO2 on 13X zeolite was higher than clinoptilolite zeolite. The ideal selectivity of CO2/CH4 for 13X and clinoptilolite zeolites are 8.47 and 5.63 at 277 K and 1 bar, respectively. The adsorption heats for the two gases on adsorbents were calculated using the van't Hoff equation. The values of adsorption heats of CO2 and CH4 for 13X zeolite are 34.65 and 15.43 kJ/mol, whereas adsorption heats of CO2 and CH4 on clinoptilolite zeolite are 21.03 and 11.62 kJ/mol, respectively. The highest adsorption heat was obtained for adsorption of CO2 on 13X zeolite, which refers to the strong adsorption of CO2 on the surface of this zeolite. The extended Langmuir and Sips models were utilized to correlate the experimental data, and Sips model showed a better fit of the data. According to the results of this study, it was found that the 13X zeolite is a more appropriate adsorbent for removing carbon dioxide from natural gas, compared with clinoptilolite. © 2015 Elsevier B.V. All rights reserved.
Keywords: Adsorption Carbon dioxide Zeolite Clinoptilolite
1. Introduction Removal of carbon dioxide from methane has become an important industrial process to improve quality of natural gas (Li et al., 2013). Natural gas contains mainly methane, other components present at natural gas are paraffinic hydrocarbons such as ethane and propane. In addition, impurities such as nitrogen and acid gas as well as trace amounts of argon and helium are found in natural gas (Mokhatab et al., 2006). The presences of acid gases like carbon dioxide, in natural gas cause corrosion and damage to pipelines. For these reasons, removal of carbon dioxide from the natural gas is very important (Doroudian et al., 2012; Li et al., 2013). Technologies employed for removal of carbon dioxide from methane are adsorption, absorption, cryogenic distillation, and membrane separation (Doroudian et al., 2012; Mulgundmath et al., 2012). Absorption process using amine as a solvent has been employed for a long time to remove carbon dioxide from natural gas. In this process, the chemical reaction that occurs between CO2 and solvent should have a high selectivity toward CO2 (Liang et al.,
* Corresponding author. E-mail address: [email protected] (S. Sharifnia). http://dx.doi.org/10.1016/j.jngse.2015.08.033 1875-5100/© 2015 Elsevier B.V. All rights reserved.
2009). In absorption process, the energy required to regenerate the solvent is considerably higher than adsorption process (Jiang et al., 2013). Adsorption due to simplicity, excellent performance and economic is an effective method for removal of carbon dioxide (Doroudian et al., 2012; McEwen et al., 2013). Adsorption can be defined as a physical or chemical process during which gas or liquid molecules are attached to the surface of the solid adsorbent to eliminate impurities. In this process, the amount of material adsorbed onto the adsorbent surface is directly related to the surface area of the solid (Mokhatab et al., 2006). Adsorbents such as activated carbon, metal organic frameworks, molecular sieves like zeolites, clay, basic resin, limestone, lithium silicate and aminemodified mesoporous silica, have been applied for CO2 removal (Mokhatab et al., 2006). Among these adsorbents, zeolites due to high thermal stability, high surface area, inexpensive, simple ion exchange, high acidity, and their tend to adsorb polar molecules are a good candidates as adsorbent in the adsorption process (Ackley et al., 2003; Arean et al., 2012; Belviso et al., 2013; Loiola et al., 2012; Martucci et al., 2012). Zeolites are a class of natural or synthetic porous crystalline alumosilicates from the alkali and alkali earth cations, especially sodium, calcium and magnesium, with three-dimensional frameworks made of SiO4 and AlO4 tetrahedral, that bridged together by sharing oxygen atoms (Elaiopoulos et al.,
A. Arefi Pour et al. / Journal of Natural Gas Science and Engineering 26 (2015) 1246e1253
2010; Ro zic et al., 2009). The adsorption properties of zeolites are dependent on several factors such as their structure, nature of extra framework cations, the number of cation and the silicon to aluminum ratio (Araki et al., 2012; Sayari et al., 2011). Clinoptilolite, mordenite, chabazite and phillipsite are instances of natural zeolites. Natural clinoptilolite belongs to the heulandite family, and its chemical formula is Na6[(AlO2)6.(SiO2)30].24H2O. The framework structure of clinoptilolite is composed of three A, B and C channels which are two-dimensional. The Both A(ten-oxygen ring) and B(eight-oxygen ring) channels are parallel together, and (eightmember ring) channel C is connected to them with a certain angle (Yang, 2003). The ratio of silicon to aluminum in the framework of clinoptilolite is in the range from 4 to 5.3. However, the ion exchange capacity of clinoptilolite is lower than the other zeolites. Typically, this zeolite can be ion exchanged with cations such as Na, K, Ca and Mg (Benkli et al., 2005; Jha and Hayashi, 2009; Sprynskyy et al., 2006). The rate of removal gas and selectivity depends on the type, number, and nature of the cation present in the channels (Jayaraman et al., 2004). Clinoptilolite, because of the uniform pore structure and molecular sieve features allows molecules with the smaller kinetic diameter such as CO2 and N2 to penetrate through the cavities, whereas molecules with a larger kinetic diameter like CH4 does not enable to penetrate into it (Ackley et al., 2003). The natural zeolites, especially clinoptilolite, because of low cost and readily available has wide applications such as ion exchanger to eliminate ammonium ion and heavy metals Arcoya et al., 1996; Jha and Hayashi, 2009; Yerli et al., 2002), removal of radioactive waste (Yang, 2003), air separation, natural gas purification, gas drying and uptake of acid gas (Ackley et al., 2003; Jayaraman et al., 2004), also they can be utilized as adsorbent in the adsorption process (Arcoya et al., 1996; Yerli et al., 2002). The synthetic zeolites such as A, X, ZSM-5, beta and Y have largest commercial application. Synthetic zeolites are more applicable than natural zeolites, which is attributed to the high degree of purity and uniform particle size of synthetic zeolites (Kulprathipanja, 2010). Zeolites such as LTA, 13X, MFI, and MOR are widely employed as adsorbent for the adsorption of carbon dioxide (Mulgundmath et al., 2012; You et al., 2013). The synthetic 13X zeolite is the sodium form of Faujasite zeolite (NaX) and is an aluminum rich zeolite. Thus, 13X zeolite has maximum cations in its structure and these cations create an electric field that interacts with quadrupole moment molecules like carbon dioxide. For this reason, 13X (NaX) zeolite is frequently employed to remove carbon dioxide from natural gas, flue gas and air (Liano-Restrepo, 2010; Silva et al., 2012). In this work, performance of clinoptilolite shaping with montmorillonite clay for separation of carbon dioxide from methane by adsorption process was compared with commercial 13X zeolite. Using this type of zeolite has been studied for the first time. The adsorption isotherms of CO2 and CH4 on adsorbents have been investigated at 277, 290 and 310 K at pressures of up to 10 bar. The extended Langmuir and Sips models have been employed to describe these isotherms. Also, the adsorption heats for CO2 and CH4 on adsorbents were determined by the van't Hoff relationship. Finally, the ability of adsorbents to remove carbon dioxide from the methane was confirmed by breakthrough experiments.
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natural zeolite was mixed with montmorillonite binder (75 wt.% clinoptilolite zeolite powder and 25 wt.% montmorillonite binder). Then, mixture is shaped into granules with a granule size of about 3 mm. The specific surface area were 534 and 84.41 m2 g1 and total pore volume were 0.29 and 0.056 cm3 g1 for 13X and natural zeolites, respectively. The chemical composition of natural zeolite was determined through X-ray fluorescence method (XRF; SPECTRO X-LabPro) and is given in Table 1. It is observed that SiO2, Al2O3 and K2O are the main components found in the sample. From the results in Table 1, it can be seen that the Si/Al ratio of natural zeolite powder is 5.02 (mol/mol), which is agreement with a Si/Al ratio reported for clinoptilolite zeolite. The X-ray diffraction analysis of natural and 13X zeolites were performed using EQUINOX diffractometer (Inel Company) with CuKa radiation (l ¼ 1.5406 Å), operated at 20 mA and 30 kV. X-ray diffraction spectrums of clinoptilolite, Muscovite, 13X and natural zeolites are given in Fig. 1. XRD pattern of the natural zeolite showed sharp peaks which correspond to clinoptilolite and its characteristic reflections were detected at 2q ¼ 9.92, 22.4 and 30 (Arcoya et al., 1996). Therefore, the main crystalline phase of natural zeolite is clinoptilolite and Muscovite which is present as traces. Also, the XRD patterns of 13X zeolite contain binder crystalline structure in 13X without impurity phase. 2.2. Gas adsorption and breakthrough experiments In order to conduct the adsorption experiments of CO2 and CH4 on 13X and natural zeolites, these adsorbents were loaded in the adsorption column. A schematic diagram of the experimental setup is displayed in Fig. 2. The height and inner diameter of the adsorption column were 62 cm and 9 mm, respectively. Height equal to 8 cm from the top and bottom of the column was filled with glass beads and the adsorbents were placed in the mid-part of the column (Doroudian et al., 2012; Won et al., 2012). The column was equipped with an electric heating jacket to regulate temperature of the adsorption column by using a thermocouple embedded in the middle of the column. The feed gas mixture contains 60% methane, 20% carbon dioxide, and 20% helium. The flow rate of feed gas was controlled using mass flow controller. Adsorption isotherms were measured at different temperatures of 277, 290 and 310 K and the pressures in the range of 1e10 bar. Adsorption experiments at 277 K were performed with the help of the ice bath, also temperature at 290 and 310 K were kept constant by a circulating water bath. Prior to each experimental run, to remove water or other impurities that may be occupied pores of zeolite; the adsorbents were activated with pure helium at a flow rate of 10 mL/ min at 673 K for 1 h, then, cooled to the room temperature. Breakthrough experiments were performed in the packed column of 13X and clinoptilolite zeolites as adsorbentat 290 K and constant column pressure of 1 bar. The feed gas was entered into the adsorption column with a flow rate of 10 mL/min. Similarly, prior to the start of the breakthrough tests, the activation of the adsorbants were conducted under helium flow at 673 K for 1 h. The concentration of the outlet gas stream of adsorption column was analyzed by using a gas chromatograph (Schroth Compact GC-CGA-1, 230 V,
2. Experimental 2.1. Adsorbent materials and characterization The natural zeolite used in this work was obtained from Semnan region of Iran. Montmorillonite K-10 clay was purchased from Fluka. The spherical beads (2.5e5 mm) form of commercial 13X zeolite was provided from Zeochem Co Switzerland. Montmorillonite K-10 clay was applied to manufacture the granules. The powder of
Table 1 Results of XRF analysis of natural zeolite (wt.%). Component
Wt.%
Component
Wt.%
SiO2 Al2O3 Na2O TiO2 Fe2O3
76.69 12.97 1.57 0.31 1.48
MgO CaO K2O P2O5 Si/Al
1.39 1.60 1.81 0.083 5.02(mol/mol)
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qi ¼
Intensity (a.u.)
Natural zeolite
Muscovite
Clinoptilolite 10
15
20
25
30
35
40
45
(2)
In this study, the parameters of the extended Langmuir and Sips models also correlation coefficients were obtained with MATLAB 7.0.
13X
5
qmi bi pni i P 1 þ nj¼1 bj pnj i
50
55
2θ Fig. 1. XRD pattern of clinoptilolite, muscovite, natural zeolites, and 13X.
2.3.2. Henry's law constant and selectivity Henry's law constant represents the affinity of adsorbate molecules toward adsorbent, and is correlated with the interaction between adsorbate and the adsorbent surface. For this reason, accurate calculation of Henry's constant is important in the adsorption process. The Langmuir equation was used to determine Henry's constant (Deng et al., 2012; Ridha and Webley, 2009). The Langmuir model at sufficiently low pressures, in Henry's law region, is reduced to linear form which is expressed by:
qi ¼ bi qmi ¼ KHi pi /0 pi lim
(3)
700 W) manufactured by Labexchange Co, and equipped with a thermal conductivity detector (TCD). The adsorptive gases applied in this work were CO2, CH4, and He with 99.5, 99.95, and 99.999% purities, respectively.
The values of b and qm in the Langmuir equation are estimated by plotting (1/q) versus (1/P) (Pakseresht et al., 2002). The relationship between temperature and Henry's constant is described using van't Hoff equation which is given as Eq. (4).
2.3. Theory and method
KHi ¼ KHoi exp
2.3.1. Isotherms equation In order to describe the adsorption equilibrium data, different models have been suggested. Some of the most common models are: Toth, Langmuir, Sips, Freundlich and UNILAN. In this work, the extended Langmuir and Sips models were applied for fitting equilibrium data. The Langmuir model is appropriate to describe the monolayer adsorption on ideal surfaces. The extended Langmuir and Sips equations are given by Eqs. (1) and (2) (Deng et al., 2012; Do, 1998; Pakseresht et al., 2002; Shao et al., 2009) [32e35].
The equilibrium selectivity at a specific temperature was calculated by using the following expression (Eq. (5)) (Deng et al., 2012).
qi ¼
qmi bi pi P 1 þ nj¼1 bj pj
(1)
Sei;j ¼
DHi RT
KHi KHj
(4)
(5)
The ideal selectivity of gas 1 over gas 2 is determined using Eq. (6) (Doroudian Rad et al., 2012).
Si;j ¼
qi =pi . qj pj
Fig. 2. Schematic diagram of the experimental setup.
(6)
A. Arefi Pour et al. / Journal of Natural Gas Science and Engineering 26 (2015) 1246e1253
P (bar)
3. Results and discussion
0
3.1. Adsorption isotherms
P (bar) 2
4
6
8
10
8 7
q (mmol/g) [CO2]
4
6
8
10
q (mmol/g) [CO2]
2.4 2 1.6 1.2 0.8 0.4 0 1.6 1.4 1.2 1
0.8 0.6
Exp(T=277 K) Exp(T=290 K) Exp(T=310 K) Sips model Langmuir model
0.4
6
0.2
5
0
0
4
2
4
6
8
10
P (bar)
3
Fig. 4. Adsorption isotherms of CO2 and CH4 on clinoptilolite.
2 1 0 3.5 3
2.5
q (mmol/g) [CH4]
2
2.8
q (mmol/g) [CH4]
The adsorption isotherms of CO2 and CH4 on 13X and clinoptilolite zeolites were measured at 277, 290 and 310 K at pressures up to 10 bar. The CO2 and CH4 adsorption isotherms on 13X and clinoptilolite are zeolites illustrated respectively in Figs. 3 and 4. According to the IUPAC classification, the isotherms of CO2 and CH4 on both adsorbents were as Type-I (Rouquerol et al., 1999). Also, the CO2 and CH4 adsorption on 13X zeolite was 7.39 mmol/g and 3.28 mmol/g at 277 K and 10 bar, respectively. Whereas, clinoptilolite can adsorb 2.8 mmol/g of CO2 and 1.39 mmol/g of CH4 under the same conditions. The adsorption capacity of CO2 is markedly higher than CH4 that refers to the preferential adsorption of carbon dioxide by zeolites compared with methane. The adsorption isotherm of CO2 on 13X zeolite is rectangular shaped that is the result of high capacity of 13X zeolite to adsorb carbon dioxide at low pressures range. In addition, the slope of the adsorption isotherms of CO2 at low pressures is much steeper than CH4, which suggests CO2 is more strongly adsorbed than CH4 on adsorbents. This can be attributed to the strong interaction of the quadrupole moment of CO2 molecules with the surface of adsorbents. CH4 is a non-polar molecule that has not dipole and quadrupole moment, and thus, interaction between CH4 and zeolites is weak (Bao et al., 2011). It is clearly seen that with increasing pressure and decreasing
0
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2 1.5
Exp(T=277K) Exp(T=290K) Exp(T=310K) Langmuir model Sips model
1 0.5 0 0
2
4
6
8
P (bar) Fig. 3. Adsorption isotherms of CO2 and CH4on 13X zeolite.
10
temperature, amount of adsorbed gas is increased. The reason for higher adsorption capacity can be interpreted by the direct relationship between temperature and kinetic energy of molecules at lower temperatures. When the temperature rises, the extra energy is produced, which leads to faster desorption of the molecules from the adsorbent surface, and the resulting achieves the equilibrium state (Munusamy et al., 2012). In addition, the effect of the adsorbent is shown in Figs. 3 and 4. As it was observed, the adsorption capacity of CO2 on 13X zeolite is greater than clinoptilolite. This is because of the larger surface area and lowers the Si/Al ratio in 13X zeolite, as compared with clinoptilolite. The main cations in clinoptilolite are potassium and calcium (Table 1), whereas sodium is main cation in 13X zeolite. According to literature (Siriwardane et al., 2003), zeolite contained a higher amount of sodium has a greater tendency to adsorb carbon dioxide. 13X zeolite possess the highest Na cation in its structure; hence it has a heterogeneous surface. The presence of these cations creates an electrostatic field which strongly interacts with quadrupole moment of molecules (Silva et al., 2012). Also, the adsorption capacity of CH4 on zeolite 13X is higher than clinoptilolite, again, it is due to the larger surface area of 13X zeolite. In order to fit the adsorption data of CO2 and CH4 on 13X and clinoptilolite zeolites, the extended Langmuir and Sips
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compared to methane. Furthermore, the equilibrium selectivity of CO2/CH4 in 13X zeolite is higher than clinoptilolite. This is attributed to the lower Si/Al ratio, and thus, the presence of more cations in the structure of 13X zeolite. The ideal selectivity of CO2 to CH4 is expressed by the ratio of the amount of CO2 adsorbed to CH4 adsorbed. Fig. 5 shows ideal selectivity of CO2 over CH4 on 13X and clinoptilolite zeolites. As can be seen, the highest value of ideal selectivity of carbon dioxide/ methane in both adsorbents is achieved at ambient pressure. At this pressure, the ideal selectivity of CO2/CH4 for 13X and clinoptilolite zeolites are 8.47 and 5.63, respectively at 277 K. The ideal selectivity of CO2/CH4 on 13X zeolite is larger than clinoptilolite. It is clearly observed that in both adsorbents, with increasing pressure, the selectivity of CO2/CH4 drops and finally, reaches to a constant value. This can be attributed to the higher tendency adsorbents to adsorb carbon dioxide on adsorbents in low pressure regions, and then, the saturation adsorption sites present on the surface of zeolites (Doroudian et al., 2012; Herm et al., 2012). As compared with other studies (Mulgundmath et al. (2012) studied), for adsorption and separation of CO2/N2 and CO2/CH4 by 13X zeolite containing binder at 313 and 373 K and pressure 4 atm, the adsorption capacity of CH4 and CO2 were equal to 1.27 and 4 mmol/g, respectively. Under the same conditions, the ideal selectivity was measured to be 3.15 and with extrapolation, the adsorption of CH4 and CO2 are measured to be 1.37 and 4.86 mmol/ g, respectively. However, in this study, the ideal selectivity is calculated to be 3.55, and the adsorption of CO2 and CH4 on 13X zeolite nearly were 22 and 8%, respectively higher than of Mulgundmath's case (almost 13% more). In the other studies (Silva et al. (2012)), the sorption and kinetics of CO2 and CH4 in binderless beads of 13X zeolite at 313 and 373 K and 4 atm were investigated. The adsorption capacity of CH4 and CO2 were equal to 1.2 and 5.2 mmol/g. Adsorption of CH4 and CO2 by extrapolation in 310 K and 4 atm were 1.23 and 5.30 mmol/g, while in this work, on 13X zeolite and in same temperature and pressure, CH4 and CO2 adsorptions were 1.4 and 5.03 mmol/g, respectively. It can be seen that the CO2 adsorption in binderless zeolite was 5% more than this study and for the case of CH4 in binderless zeolite, adsorption value was 14% lower than our results. Ideal selectivity in Silvia's work in these circumstances was 4.3 but in our work it was 3.6, therefore, our selectivity is 19% lower than Silvia's results. The adsorption heats for the two adsorbates were computed using the van't Hoff equation. These values were obtained by plotting the Ln(KH) versus reciprocal of the temperature (Pakseresht et al., 2002), as presented in Fig. 6. The adsorption heat and pre-exponential factor of CO2 and CH4 on both adsorbents are given in Table 5. As can be seen, the maximum adsorption heat belongs to adsorption of CO2 on 13X zeolite. The adsorption heat of CO2 on 13X and clinoptilolite zeolites are 34.65 and 21.03 kJ mol1, respectively. As mentioned earlier, this is because of the strong interaction between the quadrupole moments of carbon dioxide with heterogeneous surface of 13X zeolite, whereas clinoptilolite due to the presence lower amount of aluminum in its structure, it
models were applied. The extended Langmuir and Sips constants for 13X and clinoptilolite zeolites are presented in Tables 2 and 3, respectively. The b and K parameters are the affinity constant of the extended Langmuir and Sips equations, respectively. These parameters indicate how strong the adsorbate molecules are concentrated on the adsorbent surface, and there is an affinity between the adsorbent and adsorbate (Do, 1998). It can be observed that with increasing temperature, the b and K values are reduced. This suggests that an increase in temperature causes less molecules be attracted to the surface, in other words, molecules have a weaker affinity towards the adsorbent. The highest values of b and K are assigned to the adsorption of carbon dioxide on 13X zeolite, which are due to lower Si/Al ratio of 13X zeolite comparisons to clinoptilolite. As it was stated earlier, the zeolite with a lower Si/Al ratio contains more cations, and has more heterogeneous surface, and thus, has a higher affinity for carbon dioxide. Moreover, the n constant in Sips model reflects the degree of heterogeneity of the system. Similarly, values of n or the heterogeneity of the system decreases with rising temperature. In other words, the system is less heterogeneous at higher temperatures. As can be seen in Tables 2 and 3, the highest value of n belongs to the adsorption of CO2 on 13X zeolite, which represents a more heterogeneous for CO2 in 13X zeolite system. Also, the qm values decrease with increasing temperature, which proves the adsorbate loading on the surface is greater at low temperatures in comparison to high temperatures (Doroudian Rad et al., 2012). Comparison of correlation coefficients values of the extended Langmuir and Sips equations clearly indicates that Sips model is a more appropriate model to fit adsorption isotherm data. 3.2. Henry's constant and selectivity The Henry's constant of CO2 and CH4 for the different temperatures were obtained from the Langmuir equation (Ridha and Webley, 2009). The larger value of Henry's constant shows that there is a stronger interaction between gas molecules and adsorbent surface (Deng et al., 2012). Henry's constant of CO2 and CH4, and equilibrium selectivity of CO2/CH4 on 13X and clinoptilolite zeolites are given in Table 4. It is clearly observed that highest Henry's law constant belongs to adsorption of CO2 onto 13X zeolite. The reason is attributed to the stronger interaction between the quadrupole moment of CO2 molecules and more heterogeneous surface of 13X zeolite. Also, by increasing the adsorption temperature from 277 to 310 K, the Henry's constant of CO2 and CH4 gases are reduced, because of the exothermic nature of adsorption phenomena (Li and Tezel, 2007). In order to assess the separation potential an adsorbent, the equilibrium selectivity is very important parameter. This parameter is the result of differences in the affinity of adsorbent to adsorb various species present in the fluid phase (Tagliabue et al., 2009). As can be seen in Table 4, the equilibrium selectivity of CO2/CH4 for 13X and clinoptilolite zeolites decreases with increasing temperature, which is related to the higher adsorption heat of carbon dioxide
Table 2 The extended Langmuir and Sips parameters for CO2and CH4 at 277, 290 and 310 K on 13X. Adsorbate
CO2
CH4
Temp. (K)
277 290 310 277 290 310
Langmuir isotherm
Sips equation
qm (mmol g1)
B (kPa1)
R2
qm (mmol g1)
K (kPa1)
n
R2
7.6195 6.5658 5.6526 4.5389 3.7501 3.1056
0.078244 0.042608 0.021361 0.002606 0.002433 0.001943
0.9871 0.9874 0.9880 0.9944 0.9929 0.9929
7.3807 6.3084 5.375 4.6120 3.9657 3.7779
0.26396 0.070534 0.027403 0.002570 0.002319 0.001628
2.5559 1.7169 1.4012 0.9839 0.9484 0.8744
0.9996 0.9969 0.9932 0.9944 0.9930 0.9937
A. Arefi Pour et al. / Journal of Natural Gas Science and Engineering 26 (2015) 1246e1253
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Table 3 The extended Langmuir and Sips parameters for CO2 and CH4 at 277, 290 and 310 K on Clinoptilolite zeolite. Adsorbate
CO2
CH4
Temp. (K)
Langmuir isotherm
277 290 310 277 290 310
Sips equation
qm (mmol g1)
B (kPa1)
R2
qm (mmol g1)
K (kPa1)
n
R2
2.8373 2.6553 2.5063 1.9087 1.7716 1.6463
0.04316 0.02860 0.01633 0.002779 0.002047 0.001482
0.9935 0.9958 0.9953 0.9954 0.9937 0.9944
2.7868 2.6149 2.5651 2.1024 2.065 1.6968
0.05081 0.03115 0.01528 0.002548 0.001811 0.001448
1.2098 1.1170 0.9113 0.9073 0.8920 0.980
0.9944 0.9963 0.9957 0.9958 0.9943 0.9944
Table 4 Henry's constant and equilibrium selectivity for CO2 and CH4. Adsorbent
13X
Clinoptilolite
Temp.(K)
277 290 310 277 290 310
KH (mmol g1 kPa1) CO2
CH4
0.4998 0.238 0.100 0.118 0.075 0.0445
0.0103 0.0081 0.0051 0.0069 0.0052 0.00403
KH(CO2)/KH(CH4)
48.52 29.38 19.61 17.1 14.42 11.04
emerges at the output stream (Doroudian Rad et al., 2012). In this system, before reaching the breakthrough time of carbon dioxide, the effluent stream from the column was only containing methane and helium. This proves CO2 molecules are adsorbed stronger than CH4 molecules on the surface of the zeolites and the adsorbents preferentially adsorb CO2. Breakthrough time of CO2 on 13X and clinoptilolite zeolites are about 740 and 417 s, respectively. The higher adsorption capacity of CO2 on 13X zeolite compared to clinoptilolite is approved by postponement the saturated column with CO2.
possesses less heterogeneity on the surface, compared with 13X zeolite, and this refers to the strong adsorption of carbon dioxide on 13X zeolite. 3.3. Dynamic adsorption Breakthrough tests were carried out in the packed bed of 13X and clinoptilolite zeolites at 290 K and 1 bar with a feed gas flow rate of 10 mL/min. The feed gas consists of 60% methane, 20% carbon dioxide, and 20% helium. Fig. 7 presents the breakthrough curves of carbon dioxide and methane on 13X and clinoptilolite zeolites. Comparing the breakthrough curves of CO2 and CH4 on both adsorbents shows that a longer time is required for carbon dioxide to pass through the column, while this time is very shorter for methane. Breakthrough time is defined as the length of time that should be spent until the 5% of input fraction of carbon dioxide
Fig. 5. Ideal selectivity of carbon dioxide over methane for 13X and clinoptilolite zeolites at different pressures and at 277 K.
Fig. 6. Ln(KH) against of (1/T) for adsorption of CO2and CH4 on 13X and clinoptilolite zeolites.
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Table 5 Heat of adsorption and pre-exponential factors for CO2 and CH4 on 13X and clinoptilolite zeolites. Adsorbent
13X Clinoptilolite
KHo (mmol g1 kPa1)
△H (kJ/mol)
CO2
CH4
CO2
CH4
1.43E-7 1.26E-5
1.3E-5 4.36E-5
34.65 21.03
15.43 11.62
at high pressures the Sips model provides a more precise prediction of the data. (4) The longer breakthrough time confirms that the adsorbent has the higher capacity to adsorb specific species. The higher value of breakthrough time of CO2 for 13X zeolite shows that this adsorbent can be longer time actively operated for CO2 capture before regeneration process. Therefore, 13X zeolite can be a more promising adsorbent for CO2/CH4 separation.
4. Conclusions
Nomenclature
In this work, the ability of shaped clinoptilolite with montmorillonite binder and 13X zeolite as adsorbents to separate CO2 from CO2/CH4 mixture was studied. The conclusions were obtained from the current study as follows:
bi C C0 △Hi Ki KHi KH0i qi
(1) The adsorption capacity strongly depends on the surface area and pore volume of the adsorbent. Since, the 13X zeolite has high surface area and pore volume, it shows high adsorption capacity of CO2 and CH4 at all the temperatures studied. (2) The equilibrium selectivity of CO2/CH4 was affected significantly by Si/Al ratio and temperature. This factor was decreased by increasing in temperature and Si/Al ratio. (3) The heterogeneous model better describes experimental data as compared with the homogeneous model. Therefore,
qmi Pi R2 R t T ni Si,j Sei,j
Langmuir constant of component i, (kPa1) Gas concentration of component i, (mol m3) Initial concentration of component i, (mol m3) Heat of adsorption of component i, (kJ mol1) Sips constant of component i, (kPa1) Henry's constant of component i, (mmol g1 kPa1) van't Hoff parameter of component i,(mmol g1 kPa1) The amount adsorbed on the solid of component i, (mmol g1) The maximum adsorption capacity of component i, (mmol g1) Pressure, (kPa) Correlation coefficient Gas universal, (8.314 J mol1 K1) Time, (s) Temperature, (K) Sips parameter of component i, (dimensionless) The ideal selectivity of gas i over gas j, (dimensionless) The equilibrium selectivity of gas i over gas j, (dimensionless)
Abbreviations GC Gas chromatograph MFC Mass flow controller PC Personal computer TC Thermocouple References
Fig. 7. Breakthrough curves of carbon dioxide and methane on 13X and clinoptilolite zeolites at 290 K and ambient pressure.
Ackley, M.W., Rege, S.U., Saxena, H., 2003. Application of natural zeolites in the purification and separation of gases. Microporous Mesoporous Mater. 61 (1e3), 25e42. Araki, S., Kiyohara, Y., Tanaka, Sh, Miyake, Y., 2012. Adsorption of carbon dioxide and nitrogen on zeolite Rho prepared by hydrothermal synthesis using 18-crown-6 ether. J. Colloid Interf. Sci. 388, 185e190. Arcoya, A., Gonzalez, J.A., Llabre, G., Seoane, X.L., Travieso, N., 1996. Role of the countercations on the molecular sieve properties of a clinoptilolite. Microporous Mater 7, 1e13. Arean, C.O., Bibiloni, G.F., Delgado, M.R., 2012. FT-IR spectroscopic and thermodynamic study on the adsorption of carbon dioxide and dinitrogen in the alkaline zeolite K-L. Appl. Surf. Sci. 259, 367e370. Bao, Z., Yu, L., Ren, Q., Lu, X., Deng, Sh, 2011. Adsorption of CO2 and CH4 on a magnesium-based metal organic framework. J. Colloid Interface Sci. 353 (2), 549e556. Belviso, C., Cavalcante, F., Lettino, A., Fiore, S., 2013. A and X-type zeolites synthesized from kaolinite at low temperature. Appl. Clay Sci. 80e81, 162e168. Benkli, Y.E., Can, M.F., Turan, M., Celik, M.S., 2005. Modification of organo-zeolite surface for the removal of reactive azo dyes in fixed-bed reactors. Water Res. 39 (2e3), 487e493. Deng, H., Yi, H., Tang, X., Yu, Q., Ning, P., Yang, L., 2012. Adsorption equilibrium for sulfur dioxide, nitric oxide, carbon dioxide, nitrogen on 13X and 5A zeolites. Chem. Eng. J. 188, 77e85. Do, D.D., 1998. Adsorption Analysis: Equilibria and Kinetics. Imperial College Press, London. Doroudian Rad, M., Fatemi, Sh, Mirfendereski, S.M., 2012. Development of T type zeolite for separation of CO2 from CH4 in adsorption processes. Chem. Eng. Res. Des. 90 (10), 1687e1695. Elaiopoulos, K., Perraki, Th, Grigoropoulou, E., 2010. Monitoring the effect of hydrothermal treatments on the structure of a natural zeolite through a combined XRD, FTIR, XRF, SEM and N2-porosimetry analysis. Microporous Mesoporous Mater. 134 (1e3), 29e43.
A. Arefi Pour et al. / Journal of Natural Gas Science and Engineering 26 (2015) 1246e1253 Herm, Z.R., Krishna, R., Long, J.R., 2012. CO2/CH4, CH4/H2 and CO2/CH4/H2 separations at high pressures using Mg2 (dobdc). Microporous Mesoporous Mater. 151, 481e487. Jayaraman, A., Hernandez-Maldonado, A.J., Yang, R.T., Chinnb, D., Munson, C.L., Mohrb, D.H., 2004. Clinoptilolites for nitrogen/methane separation. Chem. Eng. Sci. 59 (12), 2407e2417. Jha, V.K., Hayashi, S., 2009. Modification on natural clinoptilolite zeolite for its NHþ 4 retention capacity. J. Hazard. Mater. 169 (1e3), 29e35. Jiang, Q., Rentschler, J., Sethia, G., Weinman, S., Perrone, R., Liu, K., 2013. Synthesis of Ttype zeolite nanoparticles for the separation of CO2/N2 and CO2/CH4 by adsorption process. Chem. Eng. J. 230, 380e388. Kulprathipanja, S., 2010. Zeolites in Industrial Separation and Catalysis. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany. Li, Y., Yi, H., Tang, X., Li, F., Yuan, Q., 2013. Adsorption separation of CO2/CH4 gas mixture on the commercial zeolites at atmospheric pressure. Chem. Eng. J. 229, 50e56. Liang, Z., Marshall, M., Chaffee, A.L., 2009. Comparison of Cu-BTC and zeolite 13X for adsorbent based CO2 separation. Energy Procedia 1, 1265e1271. Liano-Restrepo, M., 2010. Accurate correlation, structural interpretation, and thermochemistry of equilibrium adsorption isotherms of carbon dioxide in zeolite NaX by means of the GSTA model. Fluid Phase Equilibria 293 (1e2), 225e236. Li, P., Tezel, F.H., 2007. Adsorption separation of N2, O2, CO2 and CH4 gases by bzeolite. Microporous Mesoporous Mater. 98 (1e3), 94e101. Loiola, A.R., Andrade, J.C.R.A., Sasaki, J.M., Da Silva, L.R.D., 2012. Structural analysis of zeolite NaA synthesized by a cost-effective hydrothermal method using kaolin and its use as water softener. J. Colloid Interface Sci. 367, 34e39. Martucci, A., Pasti, L., Nassi, M., Alberti, A., Arletti, R., Bagatin, R., Vignola, R., Sticca, R., 2012. Adsorption mechanism of 1, 2-dichloroethane into an organophilic zeolite A combined diffractometric and gas chromatographic study. Microporous Mesoporous Mater. 151, 358e367. McEwen, J., Hayman, J.D., Yazaydin, A.O., 2013. A comparative study of CO2, CH4 and N2 adsorption in ZIF-8, Zeolite-13X and BPL activated carbon. J. Chem. Phys. 412, 72e76. Mokhatab, S., Poe, W.A., Speight, J.G., 2006. Handbook of Natural Gas Transmission and Processing. Elsevier, Gulf Professional Pub. Mulgundmath, V.P., Jones, R.A., Tezel, F.H., Thibault, J., 2012. Fixed bed adsorption for the removal of carbon dioxide from nitrogen: breakthrough behavior and modelling for heat and mass transfer. Sep. Purif. Technol. 85, 17e27. Munusamy, K., Sethia, G., Patil, D.V., Rallapalli, P.B.S., Somani, R.S., Bajaj, H.C., 2012.
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Sorption of carbon dioxide, methane, nitrogen and carbon monoxide on MIL101(Cr): Volumetric measurements and dynamic adsorption studies. Chem. Eng. J. 195e196, 359e368. Pakseresht, S., Kazemeini, M., Akbarnejad, M.M., 2002. Equilibrium isotherms for CO, CO2, CH4 and C2H4 on the 5A molecular sieve by a simple volumetric apparatus. Sep. Purif. Technol. 28, 53e60. Ridha, F.N., Webley, P.A., 2009. Anomalous Henry's law behavior of nitrogen and carbon dioxide adsorption on alkali-exchanged chabazite zeolites. Sep. Purif. Technol. 67 (3), 336e343. Rouquerol, F., Rouquerol, J., Sing, K., 1999. Adsorption by Powders and Porous Solids. Academic Press, London. sic, D., Sekovanic, L., Miljanic, S., Curkovic, L., Hrenovic, J., Ro zic, M., Ivanec Sipu 2009. Sorption phenomena of modification of clinoptilolite tuffs by surfactant cations. J. Colloid Interface Sci. 331 (2), 295e301. Sayari, A., Belmabkhout, Y., Serna-Guerrero, R., 2011. Flue gas treatment via CO2 adsorption. Chem. Eng. J. 171 (3), 760e774. Shao, W., Zhang, L., Li, L., Lee, R.L., 2009. Adsorption of CO2 and N2 on synthesized NaY zeolite at high temperatures. Adsorption 15 (5e6), 497e505. Silva, J.A.C., Schumann, K., Rodrigues, A.E., 2012. Sorption and kinetics of CO2 and CH4 in binderless beads of 13X zeolite. Microporous Mesoporous Mater. 158, 219e228. Siriwardane, R.V., Shen, M., Fisher, E.P., 2003. Adsorption of CO2, N2, and O2 on natural zeolites. Energy & Fuels 17 (3), 571e576. Sprynskyy, M., Buszewski, B., Terzyk, A.P., Namiesnik, J., 2006. Study of the selection mechanism of heavy metal (Pb2þ, Cu2þ ,Ni2þ, and Cd2þ) adsorption on clinoptilolite. J. Colloid Interface Sci. 304, 21e28. Tagliabue, M., Farrusseng, D., Valencia, S., Aguado, S., Ravon, U., Rizzo, C., Corma, A., Mirodatos, C., 2009. Natural gas treating by selective adsorption: material science and chemical engineering interplay. Chem. Eng. J. 155 (3), 553e566. Won, W., Lee, S., Lee, K.S., 2012. Modeling and parameter estimation for a fixed-bed adsorption process for CO2 capture using zeolite 13X. Sep. Purif. Technol. 85, 120e129. Yang, R.T., 2003. Adsorbents: Fundamentals and Applications. John Wiley & Sons, Hoboken, New Jersey. Yerli, S.Y., Ar, I., Dogu, G., Dogu, T., 2002. Removal of hydrogen sulfide by clinoptilolite in a fixed bed adsorber. Chem. Eng. Process 41 (9), 785e792. You, H.S., Jin, H., Mo, Y.H., Park, S.E., 2013. CO2 adsorption behavior of microwave synthesized zeolite beta. Mater. Lett. 108, 106e109.
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Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
Performance evaluation of clinoptilolite and 13X zeolites in CO2 separation from CO2/CH4 mixture A. Arefi Pour, S. Sharifnia*, R. NeishaboriSalehi, M. Ghodrati Catal. Res. Cen., Dept. Chem. Eng., Razi Univ., Kermanshah 67149-67246, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 April 2015 Received in revised form 14 August 2015 Accepted 17 August 2015 Available online 19 August 2015
In this study, the adsorption performance of clinoptilolite and 13X zeolites as adsorbent was examined to remove CO2 from CO2/CH4 mixture. The adsorption experiments of CO2 and CH4on 13X and clinoptilolite zeolites were conducted at 277, 290 and 310 K for pressures up to 10 bar. The adsorption capacity of CO2 on 13X zeolite was higher than clinoptilolite zeolite. The ideal selectivity of CO2/CH4 for 13X and clinoptilolite zeolites are 8.47 and 5.63 at 277 K and 1 bar, respectively. The adsorption heats for the two gases on adsorbents were calculated using the van't Hoff equation. The values of adsorption heats of CO2 and CH4 for 13X zeolite are 34.65 and 15.43 kJ/mol, whereas adsorption heats of CO2 and CH4 on clinoptilolite zeolite are 21.03 and 11.62 kJ/mol, respectively. The highest adsorption heat was obtained for adsorption of CO2 on 13X zeolite, which refers to the strong adsorption of CO2 on the surface of this zeolite. The extended Langmuir and Sips models were utilized to correlate the experimental data, and Sips model showed a better fit of the data. According to the results of this study, it was found that the 13X zeolite is a more appropriate adsorbent for removing carbon dioxide from natural gas, compared with clinoptilolite. © 2015 Elsevier B.V. All rights reserved.
Keywords: Adsorption Carbon dioxide Zeolite Clinoptilolite
1. Introduction Removal of carbon dioxide from methane has become an important industrial process to improve quality of natural gas (Li et al., 2013). Natural gas contains mainly methane, other components present at natural gas are paraffinic hydrocarbons such as ethane and propane. In addition, impurities such as nitrogen and acid gas as well as trace amounts of argon and helium are found in natural gas (Mokhatab et al., 2006). The presences of acid gases like carbon dioxide, in natural gas cause corrosion and damage to pipelines. For these reasons, removal of carbon dioxide from the natural gas is very important (Doroudian et al., 2012; Li et al., 2013). Technologies employed for removal of carbon dioxide from methane are adsorption, absorption, cryogenic distillation, and membrane separation (Doroudian et al., 2012; Mulgundmath et al., 2012). Absorption process using amine as a solvent has been employed for a long time to remove carbon dioxide from natural gas. In this process, the chemical reaction that occurs between CO2 and solvent should have a high selectivity toward CO2 (Liang et al.,
* Corresponding author. E-mail address: [email protected] (S. Sharifnia). http://dx.doi.org/10.1016/j.jngse.2015.08.033 1875-5100/© 2015 Elsevier B.V. All rights reserved.
2009). In absorption process, the energy required to regenerate the solvent is considerably higher than adsorption process (Jiang et al., 2013). Adsorption due to simplicity, excellent performance and economic is an effective method for removal of carbon dioxide (Doroudian et al., 2012; McEwen et al., 2013). Adsorption can be defined as a physical or chemical process during which gas or liquid molecules are attached to the surface of the solid adsorbent to eliminate impurities. In this process, the amount of material adsorbed onto the adsorbent surface is directly related to the surface area of the solid (Mokhatab et al., 2006). Adsorbents such as activated carbon, metal organic frameworks, molecular sieves like zeolites, clay, basic resin, limestone, lithium silicate and aminemodified mesoporous silica, have been applied for CO2 removal (Mokhatab et al., 2006). Among these adsorbents, zeolites due to high thermal stability, high surface area, inexpensive, simple ion exchange, high acidity, and their tend to adsorb polar molecules are a good candidates as adsorbent in the adsorption process (Ackley et al., 2003; Arean et al., 2012; Belviso et al., 2013; Loiola et al., 2012; Martucci et al., 2012). Zeolites are a class of natural or synthetic porous crystalline alumosilicates from the alkali and alkali earth cations, especially sodium, calcium and magnesium, with three-dimensional frameworks made of SiO4 and AlO4 tetrahedral, that bridged together by sharing oxygen atoms (Elaiopoulos et al.,
A. Arefi Pour et al. / Journal of Natural Gas Science and Engineering 26 (2015) 1246e1253
2010; Ro zic et al., 2009). The adsorption properties of zeolites are dependent on several factors such as their structure, nature of extra framework cations, the number of cation and the silicon to aluminum ratio (Araki et al., 2012; Sayari et al., 2011). Clinoptilolite, mordenite, chabazite and phillipsite are instances of natural zeolites. Natural clinoptilolite belongs to the heulandite family, and its chemical formula is Na6[(AlO2)6.(SiO2)30].24H2O. The framework structure of clinoptilolite is composed of three A, B and C channels which are two-dimensional. The Both A(ten-oxygen ring) and B(eight-oxygen ring) channels are parallel together, and (eightmember ring) channel C is connected to them with a certain angle (Yang, 2003). The ratio of silicon to aluminum in the framework of clinoptilolite is in the range from 4 to 5.3. However, the ion exchange capacity of clinoptilolite is lower than the other zeolites. Typically, this zeolite can be ion exchanged with cations such as Na, K, Ca and Mg (Benkli et al., 2005; Jha and Hayashi, 2009; Sprynskyy et al., 2006). The rate of removal gas and selectivity depends on the type, number, and nature of the cation present in the channels (Jayaraman et al., 2004). Clinoptilolite, because of the uniform pore structure and molecular sieve features allows molecules with the smaller kinetic diameter such as CO2 and N2 to penetrate through the cavities, whereas molecules with a larger kinetic diameter like CH4 does not enable to penetrate into it (Ackley et al., 2003). The natural zeolites, especially clinoptilolite, because of low cost and readily available has wide applications such as ion exchanger to eliminate ammonium ion and heavy metals Arcoya et al., 1996; Jha and Hayashi, 2009; Yerli et al., 2002), removal of radioactive waste (Yang, 2003), air separation, natural gas purification, gas drying and uptake of acid gas (Ackley et al., 2003; Jayaraman et al., 2004), also they can be utilized as adsorbent in the adsorption process (Arcoya et al., 1996; Yerli et al., 2002). The synthetic zeolites such as A, X, ZSM-5, beta and Y have largest commercial application. Synthetic zeolites are more applicable than natural zeolites, which is attributed to the high degree of purity and uniform particle size of synthetic zeolites (Kulprathipanja, 2010). Zeolites such as LTA, 13X, MFI, and MOR are widely employed as adsorbent for the adsorption of carbon dioxide (Mulgundmath et al., 2012; You et al., 2013). The synthetic 13X zeolite is the sodium form of Faujasite zeolite (NaX) and is an aluminum rich zeolite. Thus, 13X zeolite has maximum cations in its structure and these cations create an electric field that interacts with quadrupole moment molecules like carbon dioxide. For this reason, 13X (NaX) zeolite is frequently employed to remove carbon dioxide from natural gas, flue gas and air (Liano-Restrepo, 2010; Silva et al., 2012). In this work, performance of clinoptilolite shaping with montmorillonite clay for separation of carbon dioxide from methane by adsorption process was compared with commercial 13X zeolite. Using this type of zeolite has been studied for the first time. The adsorption isotherms of CO2 and CH4 on adsorbents have been investigated at 277, 290 and 310 K at pressures of up to 10 bar. The extended Langmuir and Sips models have been employed to describe these isotherms. Also, the adsorption heats for CO2 and CH4 on adsorbents were determined by the van't Hoff relationship. Finally, the ability of adsorbents to remove carbon dioxide from the methane was confirmed by breakthrough experiments.
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natural zeolite was mixed with montmorillonite binder (75 wt.% clinoptilolite zeolite powder and 25 wt.% montmorillonite binder). Then, mixture is shaped into granules with a granule size of about 3 mm. The specific surface area were 534 and 84.41 m2 g1 and total pore volume were 0.29 and 0.056 cm3 g1 for 13X and natural zeolites, respectively. The chemical composition of natural zeolite was determined through X-ray fluorescence method (XRF; SPECTRO X-LabPro) and is given in Table 1. It is observed that SiO2, Al2O3 and K2O are the main components found in the sample. From the results in Table 1, it can be seen that the Si/Al ratio of natural zeolite powder is 5.02 (mol/mol), which is agreement with a Si/Al ratio reported for clinoptilolite zeolite. The X-ray diffraction analysis of natural and 13X zeolites were performed using EQUINOX diffractometer (Inel Company) with CuKa radiation (l ¼ 1.5406 Å), operated at 20 mA and 30 kV. X-ray diffraction spectrums of clinoptilolite, Muscovite, 13X and natural zeolites are given in Fig. 1. XRD pattern of the natural zeolite showed sharp peaks which correspond to clinoptilolite and its characteristic reflections were detected at 2q ¼ 9.92, 22.4 and 30 (Arcoya et al., 1996). Therefore, the main crystalline phase of natural zeolite is clinoptilolite and Muscovite which is present as traces. Also, the XRD patterns of 13X zeolite contain binder crystalline structure in 13X without impurity phase. 2.2. Gas adsorption and breakthrough experiments In order to conduct the adsorption experiments of CO2 and CH4 on 13X and natural zeolites, these adsorbents were loaded in the adsorption column. A schematic diagram of the experimental setup is displayed in Fig. 2. The height and inner diameter of the adsorption column were 62 cm and 9 mm, respectively. Height equal to 8 cm from the top and bottom of the column was filled with glass beads and the adsorbents were placed in the mid-part of the column (Doroudian et al., 2012; Won et al., 2012). The column was equipped with an electric heating jacket to regulate temperature of the adsorption column by using a thermocouple embedded in the middle of the column. The feed gas mixture contains 60% methane, 20% carbon dioxide, and 20% helium. The flow rate of feed gas was controlled using mass flow controller. Adsorption isotherms were measured at different temperatures of 277, 290 and 310 K and the pressures in the range of 1e10 bar. Adsorption experiments at 277 K were performed with the help of the ice bath, also temperature at 290 and 310 K were kept constant by a circulating water bath. Prior to each experimental run, to remove water or other impurities that may be occupied pores of zeolite; the adsorbents were activated with pure helium at a flow rate of 10 mL/ min at 673 K for 1 h, then, cooled to the room temperature. Breakthrough experiments were performed in the packed column of 13X and clinoptilolite zeolites as adsorbentat 290 K and constant column pressure of 1 bar. The feed gas was entered into the adsorption column with a flow rate of 10 mL/min. Similarly, prior to the start of the breakthrough tests, the activation of the adsorbants were conducted under helium flow at 673 K for 1 h. The concentration of the outlet gas stream of adsorption column was analyzed by using a gas chromatograph (Schroth Compact GC-CGA-1, 230 V,
2. Experimental 2.1. Adsorbent materials and characterization The natural zeolite used in this work was obtained from Semnan region of Iran. Montmorillonite K-10 clay was purchased from Fluka. The spherical beads (2.5e5 mm) form of commercial 13X zeolite was provided from Zeochem Co Switzerland. Montmorillonite K-10 clay was applied to manufacture the granules. The powder of
Table 1 Results of XRF analysis of natural zeolite (wt.%). Component
Wt.%
Component
Wt.%
SiO2 Al2O3 Na2O TiO2 Fe2O3
76.69 12.97 1.57 0.31 1.48
MgO CaO K2O P2O5 Si/Al
1.39 1.60 1.81 0.083 5.02(mol/mol)
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qi ¼
Intensity (a.u.)
Natural zeolite
Muscovite
Clinoptilolite 10
15
20
25
30
35
40
45
(2)
In this study, the parameters of the extended Langmuir and Sips models also correlation coefficients were obtained with MATLAB 7.0.
13X
5
qmi bi pni i P 1 þ nj¼1 bj pnj i
50
55
2θ Fig. 1. XRD pattern of clinoptilolite, muscovite, natural zeolites, and 13X.
2.3.2. Henry's law constant and selectivity Henry's law constant represents the affinity of adsorbate molecules toward adsorbent, and is correlated with the interaction between adsorbate and the adsorbent surface. For this reason, accurate calculation of Henry's constant is important in the adsorption process. The Langmuir equation was used to determine Henry's constant (Deng et al., 2012; Ridha and Webley, 2009). The Langmuir model at sufficiently low pressures, in Henry's law region, is reduced to linear form which is expressed by:
qi ¼ bi qmi ¼ KHi pi /0 pi lim
(3)
700 W) manufactured by Labexchange Co, and equipped with a thermal conductivity detector (TCD). The adsorptive gases applied in this work were CO2, CH4, and He with 99.5, 99.95, and 99.999% purities, respectively.
The values of b and qm in the Langmuir equation are estimated by plotting (1/q) versus (1/P) (Pakseresht et al., 2002). The relationship between temperature and Henry's constant is described using van't Hoff equation which is given as Eq. (4).
2.3. Theory and method
KHi ¼ KHoi exp
2.3.1. Isotherms equation In order to describe the adsorption equilibrium data, different models have been suggested. Some of the most common models are: Toth, Langmuir, Sips, Freundlich and UNILAN. In this work, the extended Langmuir and Sips models were applied for fitting equilibrium data. The Langmuir model is appropriate to describe the monolayer adsorption on ideal surfaces. The extended Langmuir and Sips equations are given by Eqs. (1) and (2) (Deng et al., 2012; Do, 1998; Pakseresht et al., 2002; Shao et al., 2009) [32e35].
The equilibrium selectivity at a specific temperature was calculated by using the following expression (Eq. (5)) (Deng et al., 2012).
qi ¼
qmi bi pi P 1 þ nj¼1 bj pj
(1)
Sei;j ¼
DHi RT
KHi KHj
(4)
(5)
The ideal selectivity of gas 1 over gas 2 is determined using Eq. (6) (Doroudian Rad et al., 2012).
Si;j ¼
qi =pi . qj pj
Fig. 2. Schematic diagram of the experimental setup.
(6)
A. Arefi Pour et al. / Journal of Natural Gas Science and Engineering 26 (2015) 1246e1253
P (bar)
3. Results and discussion
0
3.1. Adsorption isotherms
P (bar) 2
4
6
8
10
8 7
q (mmol/g) [CO2]
4
6
8
10
q (mmol/g) [CO2]
2.4 2 1.6 1.2 0.8 0.4 0 1.6 1.4 1.2 1
0.8 0.6
Exp(T=277 K) Exp(T=290 K) Exp(T=310 K) Sips model Langmuir model
0.4
6
0.2
5
0
0
4
2
4
6
8
10
P (bar)
3
Fig. 4. Adsorption isotherms of CO2 and CH4 on clinoptilolite.
2 1 0 3.5 3
2.5
q (mmol/g) [CH4]
2
2.8
q (mmol/g) [CH4]
The adsorption isotherms of CO2 and CH4 on 13X and clinoptilolite zeolites were measured at 277, 290 and 310 K at pressures up to 10 bar. The CO2 and CH4 adsorption isotherms on 13X and clinoptilolite are zeolites illustrated respectively in Figs. 3 and 4. According to the IUPAC classification, the isotherms of CO2 and CH4 on both adsorbents were as Type-I (Rouquerol et al., 1999). Also, the CO2 and CH4 adsorption on 13X zeolite was 7.39 mmol/g and 3.28 mmol/g at 277 K and 10 bar, respectively. Whereas, clinoptilolite can adsorb 2.8 mmol/g of CO2 and 1.39 mmol/g of CH4 under the same conditions. The adsorption capacity of CO2 is markedly higher than CH4 that refers to the preferential adsorption of carbon dioxide by zeolites compared with methane. The adsorption isotherm of CO2 on 13X zeolite is rectangular shaped that is the result of high capacity of 13X zeolite to adsorb carbon dioxide at low pressures range. In addition, the slope of the adsorption isotherms of CO2 at low pressures is much steeper than CH4, which suggests CO2 is more strongly adsorbed than CH4 on adsorbents. This can be attributed to the strong interaction of the quadrupole moment of CO2 molecules with the surface of adsorbents. CH4 is a non-polar molecule that has not dipole and quadrupole moment, and thus, interaction between CH4 and zeolites is weak (Bao et al., 2011). It is clearly seen that with increasing pressure and decreasing
0
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2 1.5
Exp(T=277K) Exp(T=290K) Exp(T=310K) Langmuir model Sips model
1 0.5 0 0
2
4
6
8
P (bar) Fig. 3. Adsorption isotherms of CO2 and CH4on 13X zeolite.
10
temperature, amount of adsorbed gas is increased. The reason for higher adsorption capacity can be interpreted by the direct relationship between temperature and kinetic energy of molecules at lower temperatures. When the temperature rises, the extra energy is produced, which leads to faster desorption of the molecules from the adsorbent surface, and the resulting achieves the equilibrium state (Munusamy et al., 2012). In addition, the effect of the adsorbent is shown in Figs. 3 and 4. As it was observed, the adsorption capacity of CO2 on 13X zeolite is greater than clinoptilolite. This is because of the larger surface area and lowers the Si/Al ratio in 13X zeolite, as compared with clinoptilolite. The main cations in clinoptilolite are potassium and calcium (Table 1), whereas sodium is main cation in 13X zeolite. According to literature (Siriwardane et al., 2003), zeolite contained a higher amount of sodium has a greater tendency to adsorb carbon dioxide. 13X zeolite possess the highest Na cation in its structure; hence it has a heterogeneous surface. The presence of these cations creates an electrostatic field which strongly interacts with quadrupole moment of molecules (Silva et al., 2012). Also, the adsorption capacity of CH4 on zeolite 13X is higher than clinoptilolite, again, it is due to the larger surface area of 13X zeolite. In order to fit the adsorption data of CO2 and CH4 on 13X and clinoptilolite zeolites, the extended Langmuir and Sips
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compared to methane. Furthermore, the equilibrium selectivity of CO2/CH4 in 13X zeolite is higher than clinoptilolite. This is attributed to the lower Si/Al ratio, and thus, the presence of more cations in the structure of 13X zeolite. The ideal selectivity of CO2 to CH4 is expressed by the ratio of the amount of CO2 adsorbed to CH4 adsorbed. Fig. 5 shows ideal selectivity of CO2 over CH4 on 13X and clinoptilolite zeolites. As can be seen, the highest value of ideal selectivity of carbon dioxide/ methane in both adsorbents is achieved at ambient pressure. At this pressure, the ideal selectivity of CO2/CH4 for 13X and clinoptilolite zeolites are 8.47 and 5.63, respectively at 277 K. The ideal selectivity of CO2/CH4 on 13X zeolite is larger than clinoptilolite. It is clearly observed that in both adsorbents, with increasing pressure, the selectivity of CO2/CH4 drops and finally, reaches to a constant value. This can be attributed to the higher tendency adsorbents to adsorb carbon dioxide on adsorbents in low pressure regions, and then, the saturation adsorption sites present on the surface of zeolites (Doroudian et al., 2012; Herm et al., 2012). As compared with other studies (Mulgundmath et al. (2012) studied), for adsorption and separation of CO2/N2 and CO2/CH4 by 13X zeolite containing binder at 313 and 373 K and pressure 4 atm, the adsorption capacity of CH4 and CO2 were equal to 1.27 and 4 mmol/g, respectively. Under the same conditions, the ideal selectivity was measured to be 3.15 and with extrapolation, the adsorption of CH4 and CO2 are measured to be 1.37 and 4.86 mmol/ g, respectively. However, in this study, the ideal selectivity is calculated to be 3.55, and the adsorption of CO2 and CH4 on 13X zeolite nearly were 22 and 8%, respectively higher than of Mulgundmath's case (almost 13% more). In the other studies (Silva et al. (2012)), the sorption and kinetics of CO2 and CH4 in binderless beads of 13X zeolite at 313 and 373 K and 4 atm were investigated. The adsorption capacity of CH4 and CO2 were equal to 1.2 and 5.2 mmol/g. Adsorption of CH4 and CO2 by extrapolation in 310 K and 4 atm were 1.23 and 5.30 mmol/g, while in this work, on 13X zeolite and in same temperature and pressure, CH4 and CO2 adsorptions were 1.4 and 5.03 mmol/g, respectively. It can be seen that the CO2 adsorption in binderless zeolite was 5% more than this study and for the case of CH4 in binderless zeolite, adsorption value was 14% lower than our results. Ideal selectivity in Silvia's work in these circumstances was 4.3 but in our work it was 3.6, therefore, our selectivity is 19% lower than Silvia's results. The adsorption heats for the two adsorbates were computed using the van't Hoff equation. These values were obtained by plotting the Ln(KH) versus reciprocal of the temperature (Pakseresht et al., 2002), as presented in Fig. 6. The adsorption heat and pre-exponential factor of CO2 and CH4 on both adsorbents are given in Table 5. As can be seen, the maximum adsorption heat belongs to adsorption of CO2 on 13X zeolite. The adsorption heat of CO2 on 13X and clinoptilolite zeolites are 34.65 and 21.03 kJ mol1, respectively. As mentioned earlier, this is because of the strong interaction between the quadrupole moments of carbon dioxide with heterogeneous surface of 13X zeolite, whereas clinoptilolite due to the presence lower amount of aluminum in its structure, it
models were applied. The extended Langmuir and Sips constants for 13X and clinoptilolite zeolites are presented in Tables 2 and 3, respectively. The b and K parameters are the affinity constant of the extended Langmuir and Sips equations, respectively. These parameters indicate how strong the adsorbate molecules are concentrated on the adsorbent surface, and there is an affinity between the adsorbent and adsorbate (Do, 1998). It can be observed that with increasing temperature, the b and K values are reduced. This suggests that an increase in temperature causes less molecules be attracted to the surface, in other words, molecules have a weaker affinity towards the adsorbent. The highest values of b and K are assigned to the adsorption of carbon dioxide on 13X zeolite, which are due to lower Si/Al ratio of 13X zeolite comparisons to clinoptilolite. As it was stated earlier, the zeolite with a lower Si/Al ratio contains more cations, and has more heterogeneous surface, and thus, has a higher affinity for carbon dioxide. Moreover, the n constant in Sips model reflects the degree of heterogeneity of the system. Similarly, values of n or the heterogeneity of the system decreases with rising temperature. In other words, the system is less heterogeneous at higher temperatures. As can be seen in Tables 2 and 3, the highest value of n belongs to the adsorption of CO2 on 13X zeolite, which represents a more heterogeneous for CO2 in 13X zeolite system. Also, the qm values decrease with increasing temperature, which proves the adsorbate loading on the surface is greater at low temperatures in comparison to high temperatures (Doroudian Rad et al., 2012). Comparison of correlation coefficients values of the extended Langmuir and Sips equations clearly indicates that Sips model is a more appropriate model to fit adsorption isotherm data. 3.2. Henry's constant and selectivity The Henry's constant of CO2 and CH4 for the different temperatures were obtained from the Langmuir equation (Ridha and Webley, 2009). The larger value of Henry's constant shows that there is a stronger interaction between gas molecules and adsorbent surface (Deng et al., 2012). Henry's constant of CO2 and CH4, and equilibrium selectivity of CO2/CH4 on 13X and clinoptilolite zeolites are given in Table 4. It is clearly observed that highest Henry's law constant belongs to adsorption of CO2 onto 13X zeolite. The reason is attributed to the stronger interaction between the quadrupole moment of CO2 molecules and more heterogeneous surface of 13X zeolite. Also, by increasing the adsorption temperature from 277 to 310 K, the Henry's constant of CO2 and CH4 gases are reduced, because of the exothermic nature of adsorption phenomena (Li and Tezel, 2007). In order to assess the separation potential an adsorbent, the equilibrium selectivity is very important parameter. This parameter is the result of differences in the affinity of adsorbent to adsorb various species present in the fluid phase (Tagliabue et al., 2009). As can be seen in Table 4, the equilibrium selectivity of CO2/CH4 for 13X and clinoptilolite zeolites decreases with increasing temperature, which is related to the higher adsorption heat of carbon dioxide
Table 2 The extended Langmuir and Sips parameters for CO2and CH4 at 277, 290 and 310 K on 13X. Adsorbate
CO2
CH4
Temp. (K)
277 290 310 277 290 310
Langmuir isotherm
Sips equation
qm (mmol g1)
B (kPa1)
R2
qm (mmol g1)
K (kPa1)
n
R2
7.6195 6.5658 5.6526 4.5389 3.7501 3.1056
0.078244 0.042608 0.021361 0.002606 0.002433 0.001943
0.9871 0.9874 0.9880 0.9944 0.9929 0.9929
7.3807 6.3084 5.375 4.6120 3.9657 3.7779
0.26396 0.070534 0.027403 0.002570 0.002319 0.001628
2.5559 1.7169 1.4012 0.9839 0.9484 0.8744
0.9996 0.9969 0.9932 0.9944 0.9930 0.9937
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Table 3 The extended Langmuir and Sips parameters for CO2 and CH4 at 277, 290 and 310 K on Clinoptilolite zeolite. Adsorbate
CO2
CH4
Temp. (K)
Langmuir isotherm
277 290 310 277 290 310
Sips equation
qm (mmol g1)
B (kPa1)
R2
qm (mmol g1)
K (kPa1)
n
R2
2.8373 2.6553 2.5063 1.9087 1.7716 1.6463
0.04316 0.02860 0.01633 0.002779 0.002047 0.001482
0.9935 0.9958 0.9953 0.9954 0.9937 0.9944
2.7868 2.6149 2.5651 2.1024 2.065 1.6968
0.05081 0.03115 0.01528 0.002548 0.001811 0.001448
1.2098 1.1170 0.9113 0.9073 0.8920 0.980
0.9944 0.9963 0.9957 0.9958 0.9943 0.9944
Table 4 Henry's constant and equilibrium selectivity for CO2 and CH4. Adsorbent
13X
Clinoptilolite
Temp.(K)
277 290 310 277 290 310
KH (mmol g1 kPa1) CO2
CH4
0.4998 0.238 0.100 0.118 0.075 0.0445
0.0103 0.0081 0.0051 0.0069 0.0052 0.00403
KH(CO2)/KH(CH4)
48.52 29.38 19.61 17.1 14.42 11.04
emerges at the output stream (Doroudian Rad et al., 2012). In this system, before reaching the breakthrough time of carbon dioxide, the effluent stream from the column was only containing methane and helium. This proves CO2 molecules are adsorbed stronger than CH4 molecules on the surface of the zeolites and the adsorbents preferentially adsorb CO2. Breakthrough time of CO2 on 13X and clinoptilolite zeolites are about 740 and 417 s, respectively. The higher adsorption capacity of CO2 on 13X zeolite compared to clinoptilolite is approved by postponement the saturated column with CO2.
possesses less heterogeneity on the surface, compared with 13X zeolite, and this refers to the strong adsorption of carbon dioxide on 13X zeolite. 3.3. Dynamic adsorption Breakthrough tests were carried out in the packed bed of 13X and clinoptilolite zeolites at 290 K and 1 bar with a feed gas flow rate of 10 mL/min. The feed gas consists of 60% methane, 20% carbon dioxide, and 20% helium. Fig. 7 presents the breakthrough curves of carbon dioxide and methane on 13X and clinoptilolite zeolites. Comparing the breakthrough curves of CO2 and CH4 on both adsorbents shows that a longer time is required for carbon dioxide to pass through the column, while this time is very shorter for methane. Breakthrough time is defined as the length of time that should be spent until the 5% of input fraction of carbon dioxide
Fig. 5. Ideal selectivity of carbon dioxide over methane for 13X and clinoptilolite zeolites at different pressures and at 277 K.
Fig. 6. Ln(KH) against of (1/T) for adsorption of CO2and CH4 on 13X and clinoptilolite zeolites.
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Table 5 Heat of adsorption and pre-exponential factors for CO2 and CH4 on 13X and clinoptilolite zeolites. Adsorbent
13X Clinoptilolite
KHo (mmol g1 kPa1)
△H (kJ/mol)
CO2
CH4
CO2
CH4
1.43E-7 1.26E-5
1.3E-5 4.36E-5
34.65 21.03
15.43 11.62
at high pressures the Sips model provides a more precise prediction of the data. (4) The longer breakthrough time confirms that the adsorbent has the higher capacity to adsorb specific species. The higher value of breakthrough time of CO2 for 13X zeolite shows that this adsorbent can be longer time actively operated for CO2 capture before regeneration process. Therefore, 13X zeolite can be a more promising adsorbent for CO2/CH4 separation.
4. Conclusions
Nomenclature
In this work, the ability of shaped clinoptilolite with montmorillonite binder and 13X zeolite as adsorbents to separate CO2 from CO2/CH4 mixture was studied. The conclusions were obtained from the current study as follows:
bi C C0 △Hi Ki KHi KH0i qi
(1) The adsorption capacity strongly depends on the surface area and pore volume of the adsorbent. Since, the 13X zeolite has high surface area and pore volume, it shows high adsorption capacity of CO2 and CH4 at all the temperatures studied. (2) The equilibrium selectivity of CO2/CH4 was affected significantly by Si/Al ratio and temperature. This factor was decreased by increasing in temperature and Si/Al ratio. (3) The heterogeneous model better describes experimental data as compared with the homogeneous model. Therefore,
qmi Pi R2 R t T ni Si,j Sei,j
Langmuir constant of component i, (kPa1) Gas concentration of component i, (mol m3) Initial concentration of component i, (mol m3) Heat of adsorption of component i, (kJ mol1) Sips constant of component i, (kPa1) Henry's constant of component i, (mmol g1 kPa1) van't Hoff parameter of component i,(mmol g1 kPa1) The amount adsorbed on the solid of component i, (mmol g1) The maximum adsorption capacity of component i, (mmol g1) Pressure, (kPa) Correlation coefficient Gas universal, (8.314 J mol1 K1) Time, (s) Temperature, (K) Sips parameter of component i, (dimensionless) The ideal selectivity of gas i over gas j, (dimensionless) The equilibrium selectivity of gas i over gas j, (dimensionless)
Abbreviations GC Gas chromatograph MFC Mass flow controller PC Personal computer TC Thermocouple References
Fig. 7. Breakthrough curves of carbon dioxide and methane on 13X and clinoptilolite zeolites at 290 K and ambient pressure.
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